Post Treatment to Reduce Shunting Devices for Physical Etching Process

ABSTRACT

A method for etching a magnetic tunneling junction (MTJ) structure is described. A stack of MTJ layers is provided on a bottom electrode. A top electrode is provided on the MTJ stack. The top electrode is patterned. Thereafter, the MTJ stack not covered by the patterned top electrode is oxidized or nitridized. Then, the MTJ stack is patterned to form a MTJ device wherein any sidewall re-deposition formed on sidewalls of the MTJ device is non-conductive and wherein some of the dielectric layer remains on horizontal surfaces of the bottom electrode.

PRIORITY DATA

The present application is a continuation application of U.S.application Ser. No. 16/416,984, filed May 20, 2019, which is acontinuation application of U.S. application Ser. No. 15/479,514, filedApr. 5, 2017, each of which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

This application relates to the general field of magnetic tunnelingjunctions (MTJ) and, more particularly, to etching methods for formingMTJ structures.

BACKGROUND

A typical MTJ etched by a chemical etching process is found to havesidewall damage, possibly caused by oxygen or other chemicals during theetching process. Pure physical etching processes such as ion beametching (IBE) can minimize sidewall damage. However, one drawback of thephysical etching process is the sidewall re-deposition of material fromthe bottom electrode and MTJ materials to the MTJ sidewalls. Thesidewall re-deposition of the bottom electrode will lead to a shuntingpath around the MTJ sidewall and then lead to low yield for the MRAMchip.

Several patents teach methods to reduce shunting. These include U.S.Pat. No. 9,257,638 (Tan et al), U.S. Pat. No. 7,043,823 (Childress etal), U.S. Pat. No. 8,981,507 (Takahashi et al), U.S. Pat. No. 6,798,626(Hayashi et al), U.S. Pat. No. 8,045,299 (Fontana, Jr et al), U.S. Pat.No. 8,673,654 (Hong et al) and U.S. Patent Application 2016/0079308(Ito). U.S. Pat. No. 8,045,299 (Fontana, Jr et al—HGST) teaches etchingand then oxidizing the MTJ stack or adding ozone or water to the etchingprocess to oxidize the re-depositing material.

SUMMARY

It is an object of the present disclosure to provide an improved etchingprocess in forming MTJ structures.

Yet another object of the present disclosure is to provide an etchingprocess that reduces shunting of MTJ devices.

In accordance with the objectives of the present disclosure, a methodfor etching a magnetic tunneling junction (MTJ) structure is achieved. Astack of MTJ layers is provided on a bottom electrode. A top electrodeis provided on the MTJ stack. The top electrode is patterned.Thereafter, the MTJ stack not covered by the patterned top electrode isoxidized or nitridized. Then, the MTJ stack is patterned to form a MTJdevice wherein any sidewall re-deposition formed on sidewalls of the MTJdevice is non-conductive.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of thisdescription, there is shown:

FIGS. 1, 2, 3A, and 4A illustrate in cross-sectional representationsteps in a first preferred embodiment of the present disclosure.

FIGS. 1, 2, 3B, and 4B illustrate in cross-sectional representationsteps in a second preferred embodiment of the present disclosure.

DETAILED DESCRIPTION

For most pure physical etching processes (such as IBE), the sidewallalways suffers a severe re-deposition issue since the by-products of theetched material are non-volatile. To prevent the re-deposition materialaround the MTJ sidewall from becoming a shunting path for the MTJ, weapply a surface treatment by oxygen to convert the potentialre-deposition material from conductive to non-conductive. This step willensure that any re-deposition is non-conductive and will not causeshunting of the MTJ devices.

Referring now particularly to FIGS. 1 through 4, the novel disclosurewill be described in detail. A bottom electrode 12 is formed on thesubstrate 10, as shown in FIG. 1. Now, layers are deposited on thebottom electrode to form a magnetic tunnel junction. Layer 14 includesthe MTJ layers including one or more seed layers, pinned layers, tunnelbarrier layers, and free layers, as is conventional in the art. Finallya top electrode 16 is deposited on the MTJ layers 14.

A photoresist mask 25 is formed over the top electrode. As shown in FIG.2, the top electrode is patterned using the photoresist mask 25.

Now, an additional post treatment process is added in the middle of theetching process. After defining the top electrode 16 and before the mainphysical etching to define the MTJ area, preferably an oxidationtreatment 27 is performed to oxidize the entire exposed MTJ area whereinthe exposed MTJ area not covered by the patterned top electrode becomesoxidized 20 and therefore, non-conductive, as shown in FIG. 3A. That is,the entire stack not covered by the top electrode hard mask is oxidized,including the capping layer, free layer, pinned layer, seed layer and soon. This will ensure that all re-deposition after IBE etching will bestill non-conductive to prevent any shutting path.

Oxidizing the re-deposited material after etching is undesirable becausethe oxygen might damage the MTJ device. It is hard to control thepenetration depth of the oxide. Oxidizing prior to etching does notcause this problem because all of the oxygen will be gone after etching.

After the treatment process, a physical etching will be applied todefine the MTJ area, as shown in FIG. 4A. The additional treatment willnot eliminate sidewall re-deposition, but we can ensure there-deposition material 22 will not be conductive and, thus, it will notlead to a shunting path cross the MTJ barrier. Most of the etchedmaterial should be pumped out during the etching process, but even ifthere is some re-deposition on the MTJ sidewall, it will not become ashutting path because it is not conductive.

Depending on process integration, the bottom electrode could bepatterned prior to depositing the MTJ layers. Or the bottom electrodecould be patterned after patterning the MTJ device. We can eliminate there-deposition shunting problem from the bottom electrode if we increasethe oxidation power and/or time to oxidize the bottom electrode portionnot covered by the top electrode hard mask before we perform the MTJetching, as shown in FIG. 3B. Then, when we pattern the bottomelectrode, any re-deposition 22 on horizontal surfaces of the bottomelectrode layer are removed during this etching. Some re-deposition mayoccur on sidewalls of the MTJ stack, but this will be non-conductivematerial 22, as shown in FIG. 4B.

The post treatment can be applied in a variety of different ways. Thesecan include: 1) Natural oxidation or nitridation by introducing oxygenor nitrogen gas, 2) Oxidation or nitridation with plasma assist orion-beam assist, or 3) Treatment by a liquid such as water or a solvent.It might be necessary to apply the treatment multiple times to ensureall the metallic material in the MTJ stack is converted to oxide ornitride so that it becomes non-conductive.

In option 1, oxygen or nitrogen is introduced into a chamber containingthe wafer prior to MTJ etching. If the MTJ stack is not very thick, thenatural oxidation or nitridation might be enough to convert all of theMTJ stack not covered by the top electrode hard mask to a non-conductivematerial.

In option 2, plasma oxidation or nitridation might use pure O2, pure N2,or a mixture of O2 and N2. The plasma oxidation, nitridation, or mixedO2/N2 can optionally be performed with some noble gas such as Ar, Xe,and the like. O2 or N2 implantation could be performed to transform thematerial. Alternatively, O2 or N2 ion beam irradiation could performoxidation or nitridation of the exposed layer.

In option 3, water or a solvent containing —OH or —NH, for example,could convert the exposed layers to oxides or nitrides.

Since the MTJ layers are oxidized or nitridized before performing themain physical etching, there should be no remaining oxygen or nitrogengas in the area after MTJ etching is completed. This will mitigateoxygen or nitrogen damage to the MTJ sidewalls.

Although the preferred embodiment of the present disclosure has beenillustrated, and that form has been described in detail, it will bereadily understood by those skilled in the art that variousmodifications may be made therein without departing from the spirit ofthe disclosure or from the scope of the appended claims.

What is claimed is:
 1. A method comprising: providing a stack ofmaterial layers, the stack of material layers including: a bottomelectrode layer; and a magnetic tunneling junction (MTJ) stack disposedon the bottom electrode layer; converting portions of the MTJ stack anda top surface of the bottom electrode layer to a first non-conductivematerial layer; and removing the first non-conductive material layer toform a patterned MTJ stack.
 2. The method of claim 1, wherein the stackof material layers includes a top electrode layer disposed over the MTJstack, and wherein the top electrode layer is disposed over the MTJstack during the converting of the portions of the MTJ stack and the topsurface of the bottom electrode layer to the first non-conductivematerial layer.
 3. The method of claim 1, wherein the stack of materiallayers includes a top electrode layer disposed over the MTJ stack, andwherein the top electrode layer is disposed over the MTJ stack after theremoving of the first non-conductive material layer to form thepatterned MTJ stack.
 4. The method of claim 1, further comprising:forming a top electrode layer on the MTJ stack; and patterning the topelectrode layer.
 5. The method of claim 1, wherein converting portionsof the MTJ stack and the top surface of the bottom electrode layer tothe first non-conductive material layer includes performing an oxidationprocess on the portions of the MTJ stack and the top surface of thebottom electrode layer.
 6. The method of claim 1, wherein convertingportions of the MTJ stack and the top surface of the bottom electrodelayer to the first non-conductive material layer includes performing anitridation process on the portions of the MTJ stack and the top surfaceof the bottom electrode layer.
 7. The method of claim 1, whereinconverting portions of the MTJ stack and the top surface of the bottomelectrode layer to the first non-conductive material layer includesapplying oxygen and nitrogen.
 8. The method of claim 7, wherein theapplying of oxygen and nitrogen further includes applying a noble gasselected from the group consisting of Ar and Xe.
 9. A method comprising:providing a stack of material layers, the stack of material layersincluding: a bottom electrode layer; and a magnetic tunneling junction(MTJ) stack disposed on the bottom electrode layer; applying a liquid toportions of the MTJ stack to convert the portions to a firstnon-conductive material layer; and removing the first non-conductivematerial layer to form a patterned MTJ stack.
 10. The method of claim 9,wherein the liquid is water.
 11. The method of claim 9, wherein theliquid is a solvent that includes a hydroxyl group.
 12. The method ofclaim 9, wherein the liquid is a solvent that includes an amine group.13. The method of claim 9, wherein the removing of the firstnon-conductive material layer to form the patterned MTJ stack includes aportion of the first non-conductive material layer being redeposited onthe patterned MTJ stack.
 14. The method of claim 9, wherein the removingof the first non-conductive material layer to form the patterned MTJstack includes performing a physical etch process.
 15. The method ofclaim 9, wherein the MTJ stack includes a seed layer, a pinned layer, atunnel barrier layer and a free layer, and wherein at least a portion ofeach of the seed layer, the pinned layer, the tunnel barrier layer andthe free layer is converted to the first non-conductive material layerafter the applying of the liquid to the portions of the MTJ stack.
 16. Amethod comprising: forming a magnetic tunneling junction (MTJ) stackover a substrate, the MTJ stack having a top surface facing away fromthe substrate; converting portions of the MTJ stack, including the topsurface of the MTJ stack, to a first non-conductive material layer; andremoving the first non-conductive material layer to form a patterned MTJstack.
 17. The method of claim 16, wherein the first non-conductivematerial layer includes an oxide of a material of the MTJ stack, anitride of the material of the MTJ stack, or a combination thereof. 18.The method of claim 16, wherein the converting of the portions of theMTJ stack, including the top surface of the MTJ stack, to the firstnon-conductive material layer includes preforming a plasma process. 19.The method of claim 16, wherein the converting of the portions of theMTJ stack, including the top surface of the MTJ stack, to the firstnon-conductive material layer includes preforming an ion beam etchingprocess.
 20. The method of claim 26, further comprising forming a bottomelectrode on the substrate, wherein the MTJ stack is disposed over thebottom electrode, and wherein a portion of the bottom electrode isconverted to a second non-conductive material layer during theconverting of portions of the MTJ stack, including the top surface ofthe MTJ stack, to the first non-conductive material layer, and whereinthe second non-conductive material layer interfaces with the substrate.